The field of this invention is the generation of repetitive sequences of binary digits known as pseudo random sequences, also known as pseudo noise (PN), or m-sequences. Such sequences have many applications in communications because they have good statistical and autocorrelation properties. For example, they are used to scramble digital data and intelligence signals, are used as test sequences for troubleshooting or monitoring the performance of digital communications systems or media, and are used in spread spectrum systems. Such sequences have the characteristics of random data in that the number of binary 0's and binary 1's therein is close to 50-50, yet they have predicatable autocorrelation functions, either unity or 1/N, wherein N is the number of binary digits in the sequence.
The most common method of generating such PN sequences in the prior art is the Serial Shift Register Feedback (SSRF) technique. This technique can be implemented with a shift register and one Exclusive-Or (Ex-Or) gate having as inputs the outputs of a pair of shift register stages. The gate output forms the serial input of the shift register. The data is shifted down the shift register at the rate of a clock which defines the repetition rate of the resultant PN sequence. The sequence can be obtained from any one of the register stages. The sequence at each register stage will be the same but delayed by a number of bits equal to the shift register stage position. A feedback shift register of this type can generate PN sequences of up to N=2.sup.m -1 digits long, where m is the number of active register stages used. In general, a different sequence will be generated for each different pair of register stages which are connected to the inputs of the Ex-Or gate. In these prior art PN generators, if the generated sequence is to be changed, it is necessary to re-connect one or both of the gate inputs to a different shift register stage. This means that most PN generators of the SSRF type are hard-wired to generate only a single sequence or are provided with such tap-changing means as patch cords or a pair of complex programmable electronic selectors for making different connections between the register stages and the Ex-Or gate inputs.
The present invention comprises a novel method and circuitry for generating any one of a large number of PN sequences merely by appropriately programming a circuit.